The present invention relates generally to motor control circuitry and more specifically to circuitry for controlling the speed of roofing membrane welding devices in response to temperature variations.
A commonly used roofing material is heat weldable asphalt roofing membrane. The roofing membrane generally comes in rolls of various standard widths and lengths of approximately 30 feet. The asphalt roofing membrane may be applied to the roof using a hot plate welding device such as that disclosed in U.S. Pat. No. 4,743,332 to Black. In using such a device, the operator unrolls a length of roofing membrane on the roof surface and propels the hot plate welding device between the roofing membrane and the roof surface. The welding device utilizes a heating plate to heat the asphalt membrane to a liquid state in order to weld the membrane to the roof surface.
The amount of heat transferred from the heating plate to the roofing membrane is directly related to the amount of the time the membrane contacts the heating plate. Moreover, the temperature at which the membrane is applied to the roof surface is related to the ambient temperature as well as the heat applied to the membrane. The amount of heat transferred to the roofing membrane may be regulated by varying the speed at which the roof welding device moves along the roof under the membrane. As the device moves faster, the membrane spends less time in contact with the heating plate, and application temperature decreases. Likewise, as the welding device moves more slowly, the application temperature of the roofing membrane increases.
When using the hot plate welding device, it is necessary to sufficiently heat the asphalt membrane to ensure a watertight seal between the membrane and the roof surface. Moreover, since the asphalt membrane usually comprises a fabric liner inside an asphaltic or bituminous coating, excessive heating of the roofing membrane may expose or damage the fabric base, destroying the integrity of the membrane. Therefore, the operator of the roof welding device must be constantly alert so that enough heat is applied to the roofing membrane to ensure a watertight seal while at the same time excess heat is not applied which may damage the integrity of the membrane.
Heretofore, the speed of the hot plate welding device along the roof surface has been controlled manually by the operator. The operator observes the bleed out of the asphaltic or bituminous substance on either side of the roofing membrane and adjusts the speed of the device manually to ensure that the bleed out is of a sufficient, but not excessive, amount. The prior methods and devices for welding roofing membrane to roof surfaces do not automatically control the speed of the device in response to the heat being applied to the asphalt roofing membrane and thus, it is difficult for the operator to ensure that sufficient heat is applied to the roofing membrane so that a watertight seal is formed between the membrane and the roof surface, without excessive heat damaging the integrity of the membrane.
Accordingly, the present invention provides an automatic motor control for roof welding devices which controls the speed of the device across the roof surface and thus the heat applied to a roofing membrane, in response to application temperature and in response to ambient temperature.